Retarded as I am i had not seen the troop limits at the bottom of the army roster. Interesting.
I went shopping and added some infantry and archers to my roster. And upgraded a unit into irregular cavalry. I'm hoping to evolve into a somewhat cavalry heavy force.

Do you have the more than 20 units mod of mitra?
And can you tell me the other units in your army and how many?
I'm asking just by curiosity.

I have the 1.4 mod (by Mitra) (It doesn't affect the number of units on the battlefield though, but I can tell you - it's really good!)

However, since I largely disbanded some troops, I do now have 4 pikes (militia class), 3 knights, 5 sergeants, 2 Siege weapons (I disbanded all old given by quests, though, unfortunately I can now only field 2 siege weapons and I don't understand why there's such a low maximum number). Moreover, I have 2 archers militia class, and 3 Crossbows.. Plus some militia cav and so. The cap for militia at level 23 is 9. Though you can much more irregulars (If not mistaking, I think it was 1.
So basically it's 3-5-9-18 on the different caps (Lowest number for knights, highest for irregulars).